In differential geometry, a field in mathematics, a multivector field, polyvector field of degree k {\displaystyle k} , or k {\displaystyle k} -vector field, on a smooth manifold M {\displaystyle M} , is a generalization of the notion of a vector field on a manifold.
Definition A multivector field of degree k {\displaystyle k} is a global section X {\displaystyle X} of the kth exterior power ∧ k T M → M {\displaystyle \wedge ^{k}TM\to M} of the tangent bundle, i.e. X {\displaystyle X} assigns to each point p ∈ M {\displaystyle p\in M} a k {\displaystyle k} -vector in ∧ k T p M {\displaystyle \wedge ^{k}T_{p}M} . The set of all multivector fields of degree k {\displaystyle k} on M {\displaystyle M} is denoted by X k ( M ) := Γ ( ∧ k T M ) {\displaystyle {\mathfrak {X}}^{k}(M):=\Gamma (\wedge ^{k}TM)} or by T p o l y k ( M ) {\displaystyle T_{\rm {poly}}^{k}(M)} .
Particular cases If k = 0 {\displaystyle k=0} one has X 0 ( M ) := C ∞ ( M ) {\displaystyle {\mathfrak {X}}^{0}(M):={\mathcal {C}}^{\infty }(M)} ; If k = 1 {\displaystyle k=1} , one has X 1 ( M ) := X ( M ) {\displaystyle {\mathfrak {X}}^{1}(M):={\mathfrak {X}}(M)} , i.e. one recovers the notion of vector field; If k > d i m ( M ) {\displaystyle k>\mathrm {dim} (M)} , one has X k ( M ) := { 0 } {\displaystyle {\mathfrak {X}}^{k}(M):=\{0\}} , since ∧ k T M = 0 {\displaystyle \wedge ^{k}TM=0} .
Algebraic structures The set X k ( M ) {\displaystyle {\mathfrak {X}}^{k}(M)} of multivector fields is an R {\displaystyle \mathbb {R} } -vector space for every k {\displaystyle k} , so that X ∙ ( M ) = ⨁ k X k ( M ) {\displaystyle {\mathfrak {X}}^{\bullet }(M)=\bigoplus _{k}{\mathfrak {X}}^{k}(M)} is a graded vector space. Furthermore, there is a wedge product
∧ : X k ( M ) × X l ( M ) → X k + l ( M ) {\displaystyle \wedge :{\mathfrak {X}}^{k}(M)\times {\mathfrak {X}}^{l}(M)\to {\mathfrak {X}}^{k+l}(M)}
which for k = 0 {\displaystyle k=0} and l = 1 {\displaystyle l=1} recovers the standard action of smooth functions on vector fields. Such product is associative and graded commutative, making ( X ∙ ( M ) , ∧ ) {\displaystyle ({\mathfrak {X}}^{\bullet }(M),\wedge )} into a graded commutative algebra. Similarly, the Lie bracket of vector fields extends to the so-called Schouten-Nijenhuis bracket
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